I see a lot of pictures with those "line" (arc) effects in the sky and I'm wondering how this technique is called (so I can google how it's done). :
(Source: Luke Bhothipiti)
Photography Stack Exchange is a question and answer site for professional, enthusiast and amateur photographers. It only takes a minute to sign up.Sign up to join this community
Those are usually called "star trails", and they come about as a result of leaving the camera's shutter open for an extended period of time. The center of the circle will be either the north or south pole, depending on which hemisphere you are in.
As often as not, though, the sky with star trails is captured separately from the foreground, and later swapped into the foreground scene. That would be cheating, except that light pollution often makes it impossible (in a practical sense) to capture the star trails and the foreground in the same exposure unless you are in a very remote area.